Various electro-optical systems have been developed for reading optical indicia, such as bar codes. A bar code is a coded pattern of graphical indicia comprised of a matrix or series of bars and spaces of varying widths, the bars and spaces having differing light reflecting characteristics. Systems that read and decode bar codes employing CCD or CMOS-based imaging systems are typically referred to as imaging-based bar code readers or bar code scanners.
Imaging systems include CCD arrays, CMOS arrays, or other imaging pixel arrays having a plurality of photosensitive elements or pixels. Light reflected from a target image, e.g., a target bar code is focused through a lens of the imaging system onto the pixel array. Output signals from the pixels of the pixel array are digitized by an analog-to-digital converter. Decoding circuitry of the imaging system processes the digitized signals and attempts to decode the imaged bar code.
The ability of an imaging system to successfully decode an imaged bar code is directly dependent upon the ability to move the lens to a suitable position whereby a satisfactorily clear image of the target bar code is focused onto the pixel array. The imaging system focusing lens is driven by a motor, such as a piezo motor, along an axis perpendicular to the pixel array or sensor plane to permit focusing of the bar code image on the pixel array.
Whether the imaging system is housed in a handheld, portable bar code reader or a permanently mounted reader, the user of the device cannot be expected to manually focus the imaging system by moving the lens, thus, there is a need for an automatic focusing system or auto focus system for an imaging system.
Bar code imaging systems require a variable focus optical system to maximize barcode reading range and deliver high quality images over a range of distances. The high scan rate for barcode reading imposes a high-speed requirement on the auto focusing technique to be used in the imaging system.
A typical two-dimensional barcode imaging scanner has an aiming pattern generator for the user to aim the scanner at the target and a separate illuminating system for illuminating the entire two-dimensional field of view. One auto-focusing technique that uses this aiming pattern is described in the referenced parent U.S. patent application Ser. No. 10/903,792 filed on Jul. 30, 2004, which is assigned to the assignee of the present invention, and incorporated herein by reference in its entirety. The auto-focusing technique described in the '792 application uses an apparent position of a feature in the aiming pattern within the image to calculate a distance between the scanner and the target surface. In the '792 application, the aiming pattern is described as being a single dot or a pattern of dots. One common aiming pattern is a line that the user aligns so that it cuts through the entire barcode approximately perpendicular to the bars of the barcode. A two-dimensional barcode imaging scanner that includes an aiming pattern that can be also used as an illumination source for reading a one-dimensional barcode is described in U.S. patent application Ser. No. 11/262,606 filed Oct. 31, 2005, which is assigned to the assignee of the present invention, and incorporated herein by reference in its entirety.